Many popular modulation formats used in the field of digital communications assume the availability of a linear amplifier in a transmitter to boost a communication signal to a level at which it may be successfully broadcast from an antenna, then propagate to and be demodulated by a remotely located receiver. Linearity refers to the ability of an amplifier to faithfully reproduce a signal presented to it without causing the output signal from the amplifier to be distorted in some way. To the extent that the amplifier is imperfectly linear, distortion and spectral regrowth result. If this distortion and spectral regrowth are excessive, then the transmitter may fail to successfully operate within a spectral mask imposed by regulations and/or within power specifications.
Non-linearity in an amplifier causes the generation of signal harmonics as a unwanted byproduct of amplification. The even symmetry component of the non-linearity gives rise to near DC signal energy as well as to RF signal energy. Even ordered harmonics include near DC, low frequency components, collectively referred to as a video signal. The second harmonic forms a video component occupying double the baseband bandwidth, the fourth harmonic forms a video component occupying quadruple the baseband bandwidth, and so on. This envelope-induced video signal modulates amplifier gain causing further deterioration in amplifier linearity.
In some applications it is desirable to improve the power-added efficiency of the amplifier by the use of one or more variable amplifier bias signals. Such variable bias signals exhibit signal dynamics near DC, with frequency components that fall in the video signal bandwidth. They represent another form of video signal that can further modulate amplifier gain, causing still further deterioration in amplifier linearity.
FIG. 1 shows a representative amplifier portion of a conventional RF transmitter. To minimize distortion resulting from the video signal, the bias circuits and matching networks for the amplifier are conventionally configured to have as low an impedance to ground in the video band as possible. The lower the video impedance, the lower the video signal voltage, the smaller the envelope-induced bias modulation, the smaller the variable-bias-signal-induced modulation, and the smaller the distortion resulting from the video signal. In this conventional approach a number of envelope-trapping capacitors 30, often more than the three depicted in FIG. 1, have been included to implement an envelope trap by lowering the impedance in the video signal bandwidth.
FIG. 2 shows a chart of representative bias circuit impedances presented to an amplifier in accordance with a conventional approach that uses envelope trapping. Throughout the video bandwidth, one or more impedance minima 32 are presented, causing the overall impedance to remain in a low impedance range ZL. Each impedance minima 32 occurs at a resonance frequency for the network of components coupled to the HPA. The resonance frequencies are determined by the envelope-trapping capacitors 30 operating in connection with other components that are largely inductive, such as quarter-wave transmission lines (for the fundamental RF band), RF chokes, HPA package bond wires, and the like. Thus, capacitance values are chosen and envelope-trapping capacitors 30 placed in positions in the network of components where different resonance frequencies can be achieved to maintain overall video bandwidth impedance in the low impedance range ZL.
FIG. 2 also shows that in addition to low video impedance the bias circuits coupled to the HPA present a high impedance ZH throughout the fundamental RF frequency range, and impedance returns to the low impedance range ZL at higher harmonics. The high impedance range ZH exhibited in the fundamental RF band results in large part from the high impedance exhibited by the quarter-wave transmission line or RF choke in the fundamental RF band. High impedance for bias circuits in the fundamental RF band is desirable because it blocks the fundamental RF energy away from the bias circuits, causing the fundamental RF energy to flow through the output matching network and across a load RL, which exhibits a much lower impedance in the fundamental RF band. The low impedance range ZL exhibited by bias circuits at second and higher harmonics results from the presence of RF-trapping capacitors 34 (FIG. 1) to form an RF trap. This band of low impedance is desirable because it helps shunts unwanted RF energy, including higher harmonic energy, to ground, effectively removing it from the output signal and preventing it from interfering with amplifier operation.
Through the use of envelope trapping, the video signal is held to a low level, and the distortion it causes in an amplified output signal is likewise reduced. But the video signal is not eliminated, so the distortion it causes remains to some extent. And, as bandwidths increase it becomes increasingly difficult to distribute a sufficient number of envelope-trapping capacitors 30 and the resulting impedance minima 32 throughout the entire video band in a manner that keeps video impedance sufficiently low, yet also achieves a sufficiently high impedance in the fundamental RF band. When impedance in the fundamental RF band is insufficiently high, amplifier efficiency suffers.
Furthermore, several different physical characteristics of an amplifier cause different nonlinearities, with the video-signal-induced nonlinearity being only one. Another form of nonlinearity is a memory effect, where an influence of the communication signal being amplified at one instant in time may be smeared over a considerable period. In essence, an amplifier acts in part like a collection of filters, or a complicated filter, with numerous undiscovered characteristics.
Conventional efforts aimed at expanding amplifier linearization techniques to include memory effects have found only marginal success. The difficulty associated with linearizing memory effects may result from the fact that conventional amplifiers appear to exhibit many different and distinct long term and short term memory effects cross correlated with one another but each having its own unique spectral characteristics and each contributing a different degree of distortion. The difficulty may have been exacerbated by the use of envelope trapping techniques, and exacerbated further by the use extensive envelope trapping techniques to address wider signal bandwidths, because each impedance minima may be responsible for a distinct memory effect.
One of the more successful conventional efforts at addressing memory effects results from the use of a Volterra model which characterizes the actual behavior of an amplifier, with a currently popular form of this approach being called a generalized memory polynomial (GMP) model. Unfortunately, due to numerous unknown terms, a considerable amount of cross correlation between the terms, and a large span of time over which different memory effects play out, a tremendous amount of power must be consumed to derive a system of equations that model the amplifier, then take the inverse of the system of equations, and implement that inverse system of equations in signal processing hardware. Consequently, this approach is generally viewed as being unacceptable for use in battery-powered transmitters. Moreover, the tremendous processing load of this approach usually dictates that compromises be made in loop bandwidths and in precision in modeling and inversing the amplifier transfer function. Consequently, this approach typically has trouble following signal dynamics and in achieving high quality linearization results.
Another conventional effort at addressing amplifier nonlinearities, including both video-signal induced distortion and memory effects, is called envelope injection. Generally, signal processing circuits process the outgoing communication signal along with a feedback signal obtained from the output of the amplifier in an attempt to generate a baseband signal that is added to, or injected with, the amplifier biasing with the aim of canceling the video signal. But the video signal is a wideband signal that results from a complicated assortment of harmonic components acting on a component network of unknown and complicated impedance, in accordance with unknown nonlinear relationships. And, for cancellation techniques to be effective, cancellation signals should be very precisely generated. Only limited success has been achieved without employing an excessive amount of processing power to resolve the unknown parameters.
As suggested above, characterizing the actual behavior of an amplifier for use in crafting predistortion requires the consumption of a significant and often excessive amount of power. This significant power use is certainly true of the approaches based on Volterra models, but it is also true of other techniques that adapt predistortion characteristics to the particularities of a given amplifier, temperature, and other transmitter circuits. Since the spectral regrowth produced by amplifier nonlinearity covers a larger bandwidth than the bandwidth allocated to the transmitter, predistortion signals intended to linearized an amplifier likewise tend to cover a range of spectrum larger than the bandwidth allocated to the transmitter. And, feedback signals generated from the transmitter's RF output signal that are used to compensate for spectral regrowth and adapt the predistortion characteristics to the transmitter's particularities of a given instant in time also tend to be captured and processed over a larger bandwidth. Reliance on high speed, highly precise processing, including an analog-to-digital converter, and excessive use of sample speeds 4-16 times greater than the fundamental bandwidth are highly undesirable factors in many transmitters, and particularly undesirable in mass market, battery powered devices where efficient use of power and low cost are typically high priority design criteria.
Digitally capturing and processing an extra wide bandwidth signal in the feedback path is a significant contributor to power consumption and to increased costs. Accordingly, a need exists for a transmitter and method that reduces the power conventionally consumed in processing a feedback signal while nevertheless providing effective digital predistortion. And, in order for the digital predistortion to be highly effective, a need exists for a linearized transmitter and transmitter linearizing method that expand linearization efforts to address video-signal induced distortion and memory effects without consuming an excessive amount of processing power.